1. Field of the Invention
The present invention relates to a method for determining the state of charge of an electrical accumulator.
2. Description of Related Art
When electrical accumulators are used in motor vehicles, especially for electrical vehicle drives, as for instance in hybrid vehicles, exact knowledge of the state of charge of the electrical accumulator is required. Both overcharge and a high level of discharge damage the electrical accumulator and shorten its service life. For determining the state of charge of the electrical accumulator, use is made of the fact that the electrical accumulator has a voltage which varies as a function of its charge. This dependency is able to be recorded accurately if the electrical accumulator is in a load-free state.
In a first known procedure, it is provided, in determining the state of charge, that one should ascertain the voltage of the electrical accumulator and determine the state of charge from this. Starting from the state of charge, in a load operation of the electrical accumulator the electric energy withdrawn from the accumulator is detected and subtracted from the state of charge. This procedure is distinguished by a simple and efficient implementation in a calculating system that requires only a small calculating effort. In spite of the recording of the state of charge of the electrical accumulator, it is possible, in this instance, that an accumulator cell is overcharged or exhaustively discharged. This occurs when the electrical accumulator is made up of a plurality of accumulator cells, and the condition of an accumulator cell deviates from the condition of the other accumulator cells, since only the overall state of the electrical accumulator is recorded.
In a second known procedure, the voltages of all the accumulator cells are recorded and evaluated individually. Furthermore, during load operation of the accumulator, the state of charge of each individual accumulator cell is calculated. Consequently, an individual state of charge is yielded for each accumulator cell. The evaluation of the voltages of all the accumulator cells leads to a considerable effort in its implementation, such as a large computing expenditure when implemented as a computer program in a control unit.
German patent application document DE 10 2006 033 629 describes a possibility for determining the state of a battery using a short-term load of the battery using a current in the charge or discharge direction. The state is ascertained in this context with the aid of the voltage response of individual cells or groups of cells.
There is a need for recording the state of charge of an electrical accumulator completely and in a simple manner, that is able to be efficiently implemented and, in the implementation as a computer program, requires only a slight computing effort.